[Doc] Add developer notes

DevelopmentNotes.md

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+# Development Notes
+
+We welcome contributions to `guidance`, and this document exists to provide useful information contributors.
+
+## Developer Setup
+
+The quickest way to get started is to run (in a fresh environment):
+```bash
+pip install -e .[all,test]
+```
+which should bring in all of the basic required dependencies.
+Note that if you want to use GPU acceleration, then you will need to do whatever is required to allow `torch` and `llama-cpp` to access your GPU too.
+
+## Running Tests
+
+Because we run tests on GPU-equipped machines and also tests which call LLM endpoints, approval is required before our GitHub workflows will run on external Pull Requests.
+To run a basic test suite locally, we suggest:
+```bash
+python -m pytest -m "not (needs_credentials or use_gpu or server)" ./tests/
+```
+which runs our basic test suite.
+Where an LLM is required, this will default to using GPT2 on the CPU.
+To change that default, run
+```bash
+python -m pytest -m "not (needs_credentials or use_gpu or server)" --selected_model <MODELNAME> ./tests/
+```
+where `<MODELNAME>` is taken from the `AVAILABLE_MODELS` dictionary defined in `conftest.py`.
+
+## Linting
+
+We run `black` on our codebase, and plan to turn on enforcement of this in the GitHub workflows soon.